Unmanned automated guided vehicles (AGVs) that automatically travel along a predetermined circulation route have been employed in an AGV system for transporting materials, products and so on (hereinafter “objects”) within a factory or warehouse, for example. A related technology provides an AGV system in which charging devices are installed at locations for loading or unloading of objects and charge an AGV that is parked for loading or unloading of objects. Such a system eliminates the need to charge an AGV for a long time since the AGV is charged, while it is parked, with sufficient electric power for the AGV to travel from the current stop position to the next stop position. This feature of the AGV system allows the charging devices and AGVs to be produced by using smaller or lower-performance components than those typically used (such as large-capacity secondary batteries and capacitors, large-sized cooling elements and semiconductor devices, thick power cables, and large-current printed boards). Using such components is advantageous for reducing the size, weight, and cost of the charging devices and AGVs.
Another related technology is developed for transmission of electric power from a source to a load, without a direct physical connection between them. This technology, commonly called contactless power transmission (or wireless power transmission) or wireless power supply, is applicable for charging AGVs.
Unfortunately, smaller and lower-performance components tend to be less resistant to electric current, voltage, and heat, as compared with larger or higher-performance components. Thus, using power transmitters and power receivers (AGVs) beyond the designed limit may cause a damage or failure to their components, which may render it impossible to transmit or receive electric power in worst case.